The overall objective of this proposal is to identify and characterize immunoregulatory mechanisms responsible for the suppression of primary alloantigen-induced murine CTL responses. Defects in the responses and regulation of NZB CTL will be employed to help dissect normal CTL response mechanisms. Our preliminary studies suggest that both genetic constraints and active interclonal regulatory mechanisms may be important factors in the generation of primary CTL responses against the Class I-like Qa-1-encoded determinants. To further analyze our observed requirments for H-2 identity or responder and stimulator cells for generation of primary anti-Qa-1b CTL responses. (NZB x B10.BR) F1 responder cells will be stimulated with splenocytes from the congenic lines of B10 mice. The additional observation that H-2 allosensitized, irradiated spleen cells can be employed as thirdparty regulator cells to suppress a primary anti-Qa-1 CTL response have suggested that the above genetic constraints may be reflective of active interclonal regulatory mechanisms. Phenotypic fractionation of these third-party regulator cells will be used in further cell-mixing experiments to elucidate this apparently active mechanism responsible for the immunostimulatory dominance of H-2 alloantigenic determinants. We will also attempt to extend our previous studies on regulator mechanisms responsible for the suppression of H-2 alloantigen-induced CTL responses. Initial analyses demonstrated that an alloantigen-activated T cell, when restimulated with the sensitizing alloantigen, generates a soluble CTL suppressor factor (CTL-Tsf). This CTL-Tsf then mediates suppression via a plastic-adherent, radioresistent leukocyte. In the proposed studies we will attempt to characterize (CTL-Tsf by gel filtration, PAGE-SDS gel electrophoresis, and gel isoelectrophoresis techniques. Phenotypic markers will be used to further identify the sequence of mononuclear leukocyte interactions (MNL) leading to suppression of primary H-2 allo-antigen-induced CTL responses. Attempts will also be made to generate T suppressor cell clones and hybridomas. These studies should help elucidate the MNL and soluble factors which participate in the feedback suppression of CTL responses to alloantigen and provide information critical to potential efforts to therapeutically modulate alloantigenic responses.